I had the same problem with my first horse and I couldn't for the life of me figure it out until one day my trainer saw that she was always trotting on the fore hand and that threw her into that "rushed" canter and she couldn't collect and slow down. Going in a small circle should force her to start that canter the correct way so that you can set the speed and not feel outta control. If it doesn't try a much tighter circle and if she starts off throwing out into a fast canter again just slow back down and cue her again, and again and again until she gets it right. It should eventually make sense to her. I sure hope this helps you.